Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
  • H01F7/00—Magnets
  • H01F7/02—Permanent magnets [PM]
  • H01F7/0273—Magnetic circuits with PM for magnetic field generation
  • H01F7/0278—Magnetic circuits with PM for magnetic field generation for generating uniform fields, focusing, deflecting electrically charged particles
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
  • G01R33/00—Arrangements or instruments for measuring magnetic variables
  • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
  • G01R33/28—Details of apparatus provided for in groups G01R33/44 - G01R33/64
  • G01R33/38—Systems for generation, homogenisation or stabilisation of the main or gradient magnetic field
  • G01R33/383—Systems for generation, homogenisation or stabilisation of the main or gradient magnetic field using permanent magnets

Definitions

• This invention relates to yoked permanement magnets that have improved efficiency. Such magnets may be employed, for example only, in NMR equipment. BACKGROUND OF THE INVENTION
• the invention provides a yoked magnetic structure having a pair of spaced magnetized members having a permeability ⁇ 0 an ⁇ defining a cavity, and an end ferromagnetic structure closing said cavity and abutting an end of said magnetized members
• the end structure comprises at least a first element of triangular cross section having one side abutting an end of one o f said magneti z ed member and having a permeability ⁇ 0 and a second element of triangular cross section having one side abutting the adjacent side of said cavity and having a permeability ⁇ 0 .
• the first and second elements have second sides away from the magnetized member and cavity defining at least part of an equipotential line whereby the flux density on the side of the equipotential line away from the magnetized structure is zero.
• a yoked magnetic structure may be further provided with a structure of a ferromagnetic material and magnetized material positioned at an end of the cavity that permits access to the cavity whi le minimizing any inhomogeneity of the field in the cavity.
• Fig. 1 illustrates a quadrant of a cross section of a two dimensional structure, for explaining the theory of my invention
• Fig. 2 is a vector drawing of a magnetic structure designed to generate a one-dimensional flux of B, for explaining the structures of Figs. 1 and 3;
• Fig. 3 is a simplified cross sectional view of a quadrant of a magnetic structure in accordance with the invention.
• Fig. 4 is a simplified cross sectional view of a modification of the structure of Fig 3, wherein J ⁇ '"-:. " ⁇ ;
• Fig. 5 is a simplified cross sectional view of a further modification of the structure of Fig. 3, wherein J 4 and J 5 are equal to J 3 ' ;
• Fig. 6 is a perspective view of the portion of magnetic structure of the invention, for y>0, and showing one embodiment of the geometry of a yoke;
• Fig. 7 is a simplified cross sectional view of a quadrant of a magnetic structure of the type shown in Fig. 3, and further illustrating another embodiment of a yoke geometry;
• Fig. 8 is a view in accordance with Fig. 3, and further illustrating a filter in accordance with the invention.
• Fig. 9 is a view illustrating the effect of perturbations in a magnetic structure
• Fig. 10 is a simplified full cross sectional view of a magnetic structure in accordance with Fig. 8;
• Fig. 11 is a cross sectional diagram illustrating a conventional permanent magnet structure
• Fig. 12 illustrates a conventional magnet structure of clad design
• Fig. 13 is a perspective view of a part of a
• FIG. 14 is a cross sectional view illustrating a closed yoke configuration that may be employed in accordance with the invention
• Fig. 15 is a cross sectional view illustrating an open yoke configuration that may be employed in accordance with the invention.
• Fig. 16 is a simplified cross sectional view of a two dimensional magnetic structure in accordance with a further embodiment of the invention, showing the open end of the magnet;
• Fig. 17 is a view of a portion of a modification of the structure of Fig. 16;
• Fig. 18 is an end view of a known magnetic structure with a hexagonal cross section cavity
• Fig. 19 is a end view of a three dimensional magnetic structure in accordance with Fig. 16;
• Fig. 20 is a cross sectional view of a portion of the three dimensional magnetic structure taken along the lines 18-18 of Fig. 17;
• Fig. 21 is a perspective view of the magnetic element 91 of Fig. 17;
• Fig. 22 is a perspective view of the magnetic element 100 of Fig. 17;
• Fig. 23 is a perspective view of the end of a magnetic structure in accordance with Figs. 17-20. DETAILED DISCLOSURE OF THE INVENTION
• the figure of merit M of a magnetic structure having a cavity is defined by the equation:
• a m where A c and A m are the cross sectional areas of the cavity of of the magnetic material, respectively, and K is a parameter defined as: H 0 K - ⁇ 0 (2)
• H 0 is the magnitude of intensity H 0 within the cavity and J is the uniform remanence in the magnetic material.
• Fig. 2 illustrates a vector diagram illustrating the conditions existing in the arrangement of Fig. 1.
• Fig. 2 is a particular case of the general matter of determining the magnetization and the geometry of each component of a magnetic structure.
• Circle C- ⁇ has a diameter equal to the magnitude of the magnetization J lf the vector -i extending upwardly from a point N to a point N 0 .
• a point A is located on the line N-N 0 , and the and the vector A-N 0 on this line corresponds to the field ⁇ H 0 being generated in the cavity.
• the vector AN is oriented downward, and this vector is equal to J j ⁇ - ⁇ Hg so the vector AN is equal to J - ⁇ H 0 .
• a quadrant of the type illustrated in Fig. 1 is provided with a first triangular cross section element 11 of magnetic material positioned with one side 12 abutting the end of the magnetized material 10.
• a second triangular cross section element 13 is positioned with one side 14 thereof closing the open end of the cavity 15, and a second side abutting a side of the triangular cross section element along a line 16.
• the third side 17 of the element 11, and the third side 18 of the element 13 define an external interface, which, in accordance with the invention, defines an equipotential line along with the outer surface - ⁇ of the magnetic material 10.
• the line 16 extends at an angle ⁇ to the interface between the magnetiized material and the cavity 15, and hence also to the equipotential plane 20 at the center of the magnetic structure.
• the permeability of the magnetized material 10 is ⁇ -
• the permeability of the element 11 is ⁇ 2
• the permeability of the element 13 is ⁇ 3 .
• the magnetic field within the magnetized material 10 is a vector A-N, which is (1-K -L) , in Fig.
• J 3 is the magnetization of the component of magnetized material that interfaces with the cavity. J 3 is determined by the vector diagram inscribed in the circle c 3 . Because the induction is assumed to be zero in this element, which interfaces with the cavity, the magnetic field in this element must be equal and opposite to J 3 . To satisfy the condition of continuity of the magnetic field across the interface with the cavity, the tip A 3 of vector -J 3 must be on the line perpendicular to J that passes through point N 0 .
• a vector J 2 of magnitude identical to that of J- ⁇ is constructed by extending from the point A 2 to point A.
• the vector J 2 is the diameter of a circle C 2 .
• a vector J 3 of the same magnitude vector J extends downwardly from A 3 to point A. This vector J 3 is the diameter of circle C 3 .
• the line 16 (Fig. 3) separates the two triangular cross section elements 11 and 13. This line must extend in such a direction that the tangential component of the two vectors J 2 and J 3 on this line is continuous across the surface. In order to obtain this result, the line 16 must extend parallel to the line defined by the point A in Fig. 2, and the intersection of circle C 2 and C 3 .
• the outer surfaces 17, 18 of the two triangular cross section elements is perpendicular to J 2 and J 3 respectively, and is an equipotential line since, being perpendicular to J means that they are perpendicular to H. As a result, the lines 17, 18 are equipotential.
• the structure is symmetrical with respect to the equatorial plane, as well as about the line O-V 0 .
• the magnetic circuit may be closed by a yoke that is positioned anywhere. The yoke may be spaced from the surface 17, 18, or it may be adjacent the surface 17,
• a vector J 3 ' can be drawn between the point A and the perpendicular to line N-N 0 at point N 0 , that is shorter than the vector J 3 , thereby corresponding to a lower cost material than that represented by the vector J 3 .
• the vector J 3 • defines the diameter of a circle C 3 • .
• the line joining the point A and the intersection of the circles C and C 3 • extends in a direction that defines the interface between the corresponding triangular cross section magnetic elements 20, 21, as seen in Fig. 4.
• the flux of B is uniform and parallel to the y axis, and
• the calculation of the yokeless magnetic structure to be inserted between line (V- ⁇ S 2 ) and the surrounding medium is given by the vector diagram of Fig. 2. If the remanence of the components of this structure is equal to the magnitude of J ⁇ , the geometry of the structure is provided by the vector diagrams inscribed in the two circles c 2 and c 3 that pass through points A, N and A, N 0 respectively, as indicated in Fig. 2. The resulting structure is shown in Fig. 3. The remanence of the magnetic material is regions (V- ⁇ S ⁇ U ⁇ and (S- ⁇ S 2 U ⁇ are vectors J 2 , 3 whose tips coincide with point A in Fig . 2 .
• a closed boundary of the external yoke is necessary to close the flux of the magnetic induction which crosses line U-L in Fig. 1.
• any arbitrary closed line can be chosen outside boundary (V ⁇ U ⁇ S 2 ), as for instance the rectangular geometry shown in Fig. 3.
• line (V 0 V ⁇ ⁇ S ) could be the interface between magnetic structure and yoke.
• the magnet configuration of Fig. 3 makes it possible to generate the one-dimensional flux of induction B within both cavity and magnetic structure as well.
• the figure of merit of the two-dimensional structure of Fig. 3 is
• M approaches the figure of merit of an ideal, one dimensional yoked magnet whose value is given by
• the two triangular regions of Fig. 3 with remanences J ,J do not contribute to the energy of the field inside the cavity. Thus it may be of practical interest to modify the structure of Fig. 3 by using materials with lower remanences to design the two triangular regions. This is possible as long as the two regions have remanences J 2 , J 3 of magnitudes
• the vector diagram of Fig. 2 provides the change of the geometry of the magnetic structure resulting from a selection of remanences
• the invention readily enables the use of a lower cost material for all or part of the closing structure of the magnet, it is practical to compute a figure of merit in terms of the cost of the structure, rather in terms of the energy.
• the yoke for channeling the flux touches the equipotential surface defined by the triangular cross section elements, or whether a gap is provided between these surfaces and the yoke.
• the end structure consist only of two triangular cross section elements.
• additional elements may be provided.
• the permeabilities and angle of the interfaces of these elements is determined in the same manner as in the case of the structures of Figs. 3 and 4.
• the yoke 30 has a zero thickness at the center of the magnetic structure, at point 31.
• the thickness of the yoke increases toward the end of the structure, and the same thickness is maintained in the portions of the yoke adjacent the triangular cross section elements 11 and 13.
• the interface 32 between the portion of the yoke abutting the magnetized material 10 and that abutting the element 11, extends at an angle from the point V ⁇ that bisects the lines U ] _ and 17.
• the interface between the portions of the yoke adjacent the elements 11 and 13 extends parallel to the line 16.
• the closing of the flux of B within an equipotential region of the magnet can be achieved with the use of a high permeability ferromagnetic yoke, or alternatively such a high permeability yoke can be replaced by a structure of magnetic material where:
• the yoke 30 has a zero thickness at the center of the magnetic structure, at point 31.
• the thickness of the yoke increases toward the end of the structure, and the same thickness is maintained in the portions of the yoke adjacent the triangular cross section elements 11 and 13.
• the interface 32 between the portion of the yoke abutting the magnetized material 10 and that abutting the element 11, extends at an angle from the point V ⁇ that bisects the lines u ⁇ and 17.
• the interface between the portions of the yoke adjacent the elements 11 and 13 extends parallel to the line 16.
• the heavy arrows in the cross hatched area indicate the orientations of the remanences, and these remanences are parallel to the external boundary of each component of the structure.
• the geometry of Fig. 7 corresponds to the selection of a remanence J within the cross hatched area of Fig. 7 that is equal in magnitude to the remanence J ⁇ of the rectangular region (V 0 V ⁇ S S 0 ) .
• Fig. 7 shows the external boundary of yokeless, single layer magnet designed in accordance with the method developed in the article "Yokeless Permanement Magnets", M.G. Abele, T.R.-14, New York University, N.Y., Nov. 1, 1986.
• This magnet is designed around the rectangular cavity (0 S 0 S ] ⁇ S ) for the same value of K.
• the difference in total area between the two magnetic structures is apparent.
• the yokeless magnet, wherein each component generates and channels the flux of B, requires a smaller volume of magnetic material.
• Fig. 8 therein is illustrated the quadrant of the m ⁇ _netic structure of Fig. 3, showing a plurality of equipotential lines 40 extending through the magnetized material 10, the cavity closure elements 11 and 13, and the cavity 15. In the absence of any perturbations in the magnetic structure, these lines are straight and parallel, as illustrated, and provide a uniform fiels H 0 in the cavity.
• the magnetism is not perfectly uniform, for example a perturbation 41 in the magnetized material as illustrated in Fig. 9.
• the perturbation may have a larger or smaller magnetization than the remainder of the material.
• the field generated by this perturbation changes the local mangetization, and the material of the magnetized material is transparent to this field.
• the perturbation generates a dipole field that extends throughout the structure. As a consequence, the field in the cavity is no longer uniform.
• a layer 50 of high permeability material is provided at the interface between the magnetized material 10 and the cavity.
• This layer has a uniform thickness, and may have a ⁇ of, for example, 4 or 5 thousand. Due to the use of this layer 50 of high permeability, the effect of the field of the perturbation on the field in the cavity is very small.
• the layer 50 acts as a shield or filter to minize the effect of the field of the perturbation on the field in the cavity. Since the layers 50 have a uniform thickness, they do not serve to shape the field, the shape of the field being the same either with or without them.
• the field configuration is not perturbed if an equipotential surface is assumed to be a surface of infinite magnetic permeability.
• This surface may be an open surface with an arbitrary boundary, or it may be closed, in which case a region of the field is removed from the structure.
• FIG. 10 shows the lines of flux of B in the cross-section of the magnetic structure. Since B is identically zero in the regions of remanence J 2 and J 3 , the design of the two-dimensional structure is readily extended to the design of a three dimensional yoked magnet with a unidirectional flux of B.
• Fig. 13 The external boundary of the three-dimensional magnetic structure enclosing a rectangular prismatic cavity is shown in Fig. 13 in the half space y > 0.
• the schematic of the yoke is shown in Fig. 6 again the half space y > 0.
• the yoke can be partially open because no field is present in the region between the yoke and the magnetic structure.
• Figs. 14 and 15 show the schematics of closed and open yoke configurations that may be employed, in accordance with the invention.
• the uniform field within the rectangular cavity (0 S 0 SI S 2 ) can also be generated with the known hybrid magnetic structure of Fig. 12 where the magnetic material in the rectangular region (S T- ⁇ U 2 S 2 ) has a remanence J- ⁇ oriented in the direction opposite to H 0 with a magnitude:
• the area of the cross-section of the region of remanence J ⁇ is:
• a magnet design following the approach of Fig. 12 is known as "cladding" ("Applications of Cobalt-Samarium Magnets to Microwave Tubes", W. Newgebauer and E.M. Branch, General Electric Co., Schenectady, N.Y., March 15, 1972; J.P. Clarke and H.A. Leupold, IEEE Trans. Magn. MAG-22, No. 5, p 1063, 1986; and H.A. Leupold and E. Potenziani, IEEE Trans. Magn. MAG-22, No.
• Fig. 11 is a cross sectional view of a conventional axisymmetric magnet with a closed cylindrical yoke, designed to generate a uniform field in a region of the gap about its center.
• the essential difference lies in the fact that in the arrangement of the invention, as illustrated in Fig. 10, the magnetic material is a closed structure that totally encloses the cavity 15 of the magnet, while in the conventional arrangement of Fig. 11 there is no physical separation of the cavity from the yoke.
• the closed magnetic structure of Fig. 10 eliminates the fringe field area of the conventional magnet of Fig. 11 where a large fraction of the total flux generated by the magnetic material is wasted in the volume outside of the region of interest.
• the region of field uniformity is the full volume of the prismatic cavity, while in the conventional magnet, the field uniformity with the region of interest is achieved at the cost of a larger volume of the region between the pole pieces.
• the magnetic material also operates at the peak of the energy product, and the geometry of the structure is designed to generate the selected value of parameter K with the region of interest.
• the volume of magnetic material of the magnet of Fig. 7 is smaller than the volume of the closed structure of Fig. 3. However the volume of uniform field in Fig. 7 is only a fraction of the volume of the cavity of Fig. 3.
• the closed magnetic structure of Fig. 3 must be partially open to the external medium.
• the opening introduces a decrease of the intensity, as well as a distortion of the field within the cavity, that can be partially compensated by the techniques discussed in "Properties of the Magnetic Field in Yokeless Permanent Magnets", M. G. Abele, T.R.- 19, New York University, N.Y., July 1, 1988.
• a magnetic structure having a cavity extending along a Z axis of the structure, being fully enclosed on its sides, but having an open end for receiving objects.
• the field in the center of the cavity is not homogeneous unless the length of the structure is very large as compared with its transverse dimensions.
• a finite length of the field was compensated in a region of interest by providing a series of transverse cuts in the core of the magnetized material.
• the cuts greatly increased the inhomogeneity of the field immediately outside of the compensated region, and also decreases the strength of the field in the region of interest.
• the solution was not available for use in magnetic structures wherein the length of the core was not significantly greater than the transverse dimensions thereof (e.g. at least 3 to 4 times the transverse dimensions of the core) .
• Fig. 16 which depicts in a simple manner the details of a magnetic structure close to an open end of a magnet of the type illustrated in Fig. 1, having a cavity 15 extending along the Z axis 90, it has been found that significant improvement can in obtained in the homogeneity of the field in the cavity by providing triangular cross section magnetized elements 91 abutting the end of each of the elements 10 of magnetic material, e.g. the same material as the elements 10, along the end surfaces 92 thereof.
• the triangular elements 91 have sides 93 toward the Z axis 90, and sides 94 away from the Z axis 90.
• the sides 93, 94 join one another along junctions 95, the edges being closer to the Z axis than the magnetic elements 10.
• the triangular elements 91 are of magnetized material, and have magnetizations J 5 perpendicular to their sides 94, with the magnitude of the magnetizations J 5 determined by a vector diagram in accordance with Fig. 2. Therefore the magnitude of J 5 is equal and opposite to the magnetic field within the triangular cross sections elements 91.
• the entire outside surface of the magnetic str ⁇ ture is equipotental at zero potential, and may be covered with a magnetic yoke 96.
• Fig. 19 and 20 are an end view and a longitudinal cross sectional view, respectively, of a magnetic structure in accordance with Fig. 16, positioned on a magnetic structure, which may be of known configuration, having a hexagonal cross section cavity 15.
• Fig. 18 is an end view of such a known magnetic structure.
• the upper and lower sides of the cavity are defined by the magnetized members 10, and magnetized triangular cross section members 120, 121, 122 and 123 have inner surfaces defining the remainder of the sides of the cavity.
• the outer periphery of this structure may be rectangular, as illustrated, with air or material of similar magnetic characteristics being provided in the triangular cross section regions 126 between the members 10 and the adjacent members 121 - 123.
• Figs 19 amd 20 is thus adapted to be positioned on the end of a core of a magnetic structure such as illustrated in Fig. 18.
• the sides 101 of the elements 91 with triangular cross sections abut the ends of the magnetic elements 10 and are generally trapezoidal (see Fig. 21) , having long edges 102 abutting the outer transverse edges of the magnetic elements 10 and shorter parallel edges 103 abutting the inner edges of the magnetic elements 10 adjacent the cavity.
• the edges 95 of the elements 91 have the same length as the edges 103. Accordingly, the triangular faces 104 of the elements 91 extend in planes that are at non-right angles to the edges 95 and 102.
• triangular pyramidal magnetized magnetic elements 100 are provided at the end of the cavity and having bases 110 (see Fig. 22) abutting separate ones of the magnetized elements 120, 121, 122, 123 without overlapping the cavity. These magnetic elements have triangular surfaces 111 of the same shape as the sides 104 of the elements 91 that abut the sides 104.
• the elements 100 have inner triangular sides 112 facing the cavity 15 and outer triangular sides 114.
• the elements 100 have magnetizations extending perpendicular to the sides 114 thereof, the magnitudes of the magnetizations being determined by a vector diagram in accordance with Fig. 2.
• Fig. 23 illustrates a perspective view of an end of a magnetic structure in accordance with Figs. 16-20, in order to enable a clearer appreciation of the invention.

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• Physics & Mathematics (AREA)
• Electromagnetism (AREA)
• Engineering & Computer Science (AREA)
• Power Engineering (AREA)
• Condensed Matter Physics & Semiconductors (AREA)
• General Physics & Mathematics (AREA)
• Hard Magnetic Materials (AREA)
• Permanent Field Magnets Of Synchronous Machinery (AREA)
• Transmission And Conversion Of Sensor Element Output (AREA)
• Particle Accelerators (AREA)

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• 1991
  • 1991-05-30 US US07/707,620 patent/US5162771A/en not_active Expired - Lifetime
• 1992
  • 1992-04-06 WO PCT/US1992/002766 patent/WO1992022076A1/en active IP Right Grant
  • 1992-04-06 EP EP92914065A patent/EP0586602B1/en not_active Expired - Lifetime
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